1. Field of the Invention
This invention relates to an accurate mass-produced, flat, multiple-beam load cell for use in weighing devices in commercial, industrial, medical, office, home and other applications--where it may be necessary or desirable to considerably reduce the overall profile of the weighing device at a low cost.
2. Description of the Prior Art
Low profile electronic scales have a number of distinct advantages over thicker scales. In industrial or warehousing applications, for example, they do not require special cavities on shop floors or long ramps to mount forklifts or dollies onto the scale platform. In commercial applications, they help create more ergonomic designs in point-of-sale locations. In medical, office and home applications, they make it possible to design lighter and more portable scales which conserve space.
Most mass-produced low-profile electronic scale designs usually use a rigid load bearing platform which is placed on a plurality of load cells. The electrical signals from the load cells are summed to obtain an accurate measure of the total load on the platform. The overall thickness of such scales is largely determined by the thickness of the load cells. Very few commercial load cells utilizing this principle attain a truly low profile--say a 1/4-inch (6.3 mm) in thickness for a scale with a 1,000 lb. (450 kgs.) capacity, or less than 1/8-inch (3.1 mm.) thickness for a bathroom scale, point-of-sale scale, or a baby incubator scale--while maintaining a high level of accuracy at a low cost.
One such load cell is described in U.S. Pat. No. 4,993,506, issued to Angel, and entitled "Mass-Produced Flat One-Piece Load Cell and Scales Incorporating It". This type of load cell, however, does not perform well when subjected to lateral or horizontal forces or to eccentric vertical forces. Such forces may bend the flexure beam horizontally or rotate it about its own axis, thereby creating distortions which reduce the accuracy of the load cell, particularly in low-capacity applications.
Furthermore, this load cell is not suitable to scales with very rigid low-profile requirements. This is because the overall deflection of the single flexure beam and the deflection and rotation of the U-shaped elements are considerable, requiring considerable vertical space within the scale platform. Also, in most applications, the load-receiving U-shaped elements require a bridge (see, for example, FIG. 7 of the Angel Patent) connecting their two edges so as to concentrate the load in the center of the flexure beam. This bridge requires some thickness as well as some clearance away from the flexure beam which adds to the overall thickness of the scale.
In order to improve the performance of a single-flexure-beam load cell under lateral forces, the number of flexure beams in the horizontal plane may be increased to two. By increasing the number of flexure beams in the horizontal plane, while ensuring that they are parallel and symmetrical with respect to the main axis of the load cell, it is possible to overcome the effect of lateral and eccentric forces. U.S. Pat. No. 4,128,001, issued to E. A. Marks, and entitled "Parallel Beam Load Cell Insensitive to Point of Application of Load," discloses parallel flexure beams in the vertical plane, which, when under load, bend into double-cantilever S-shapes, ensuring that one side is under tension and one under equal-and-opposite compression. When such a parallel-flexure-beam arrangement is applied in the horizontal plane it largely eliminates the effect of lateral forces, as well as preventing the flexure beams from twisting about their own axes.
In other prior devices, a parallel-beam arrangement is used in the horizontal plane. Knothe et al., U.S. Pat. No. 4,542,800, entitled "Spring Body with Integrated Parallel Guide for a Balance with an Upper Scale", discloses a load cell element with four parallel flexure beams, two in each of two horizontal planes. This arrangement requires, however, a significant vertical thickness in order to permit a parallel-beam arrangement in two horizontal planes and a vertical spacing between each horizontal beam plane (see FIG. 2 of Knothe). Since two horizontal planes and a vertical spacing therebetween are required, this type of load cell is not suitable for making very thin load cells.
Kastel, U.S. Pat. No. 4,548,086, entitled "Deflecting Spring", discloses a deflecting spring particularly suited for use in pressure or force gauges. This disclosure requires a closed-perimeter clamping section (6), which is then inserted into a gauge (15), say a pressure gauge (FIG. 3). To provide a closed perimeter clamping section (6), two (U-shaped) slots (18) and (19) are required. The use of a closed perimeter and two U-shaped slots results in a larger and wider clamping section which is subject to bending and twisting forces and requires a larger horizontal area in order to be installed. The Kastel device relies on concentrating the load at the center of the spring (see, for example, FIG. 3), and the strain transducers are mounted on the flexure beams (10), (11), (12) and (13) in close proximity to the transverse members (7) and (20) connecting them, and then only on one side of the flexure beams. This arrangement is not suitable for a load cell application, say in a scale, where lateral or eccentric vertical forces are usually present, and where the load may not be centered. Such forces will cause a bending or twisting of the transverse members (7) and (20) which would cause the readings of the strain gauges to vary asymmetrically due to their close proximity to the transverse members. Indeed, Kastel does not suggest in his disclosure that his spring can function as a load cell in a weighing device.
The limitations of the prior art discussed above which make it difficult to construct an accurate load cell for use in minimum-profile scales and other weighing devices are overcome in the present invention. Accordingly it is an object of the present invention to increase the accuracy and to reduce the overall profile of scales and weighing devices incorporating load cells. By increasing the number of flexure beams in the horizontal plane, while ensuring that they are symmetrical with respect to the main axis of the load cell, it is possible to overcome the effect of lateral and eccentric forces. The introduction a lateral force bends the flexure beams in a manner whereby its effect is cancelled, and the plurality of parallel beams ensures that the introduction of an eccentric force creates bending in the beams rather than rotating them about their axes.